Expandable interbody fusion device

ABSTRACT

An expandable interbody fusion device includes superior and inferior plates that are configured to receive a sequentially inserted stack of expansion members or wafers. The superior and inferior plates include features that at least initially interlock the two plates until the superior plate is dislodged by pressure from the growing wafer stack. The wafers include features on their top and bottom surfaces that interlock the wafers in multiple degrees of freedom so that the wafer stack is not disrupted when the fusion device is fully expanded. Each wafer also includes features that interlock with the inferior plate until the wafer id dislodged by sequential introduction of another wafer.

REFERENCE TO RELATED APPLICATION

This application claims priority to co-pending provisional applicationNo. 60/604,422, filed on Aug. 25, 2004, and entitled “ExpandableInterbody Fusion Device”. The disclosure of this provisional applicationis incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to devices and methods for distraction andstabilization of tissue surfaces, and most particularly forstabilization of the intervertebral disc space.

The number of spinal surgeries to correct the causes of low back painhas steadily increased over the last several years. Most often, low backpain originates from damage or defects in the spinal disc betweenadjacent vertebrae. The disc can be herniated or can be suffering from avariety of degenerative conditions, so that in either case theanatomical function of the spinal disc is disrupted. The most prevalentsurgical treatment for these types of conditions has been to fuse thetwo vertebrae surrounding the affected disc. In most cases, the entiredisc will be removed, except for the annulus, by way of a discectomyprocedure. Since the damaged disc material has been removed, somethingmust be positioned within the intra-discal space, otherwise the spacemay collapse resulting in damage to the nerves extending along thespinal column.

In order to prevent this disc space collapse, the intra-discal space hasbeen filled with bone or a bone substitute in order to fuse the twoadjacent vertebrae together. In early techniques, bone material wassimply disposed between the adjacent vertebrae, typically at theposterior aspect of the vertebrae, and the spinal column was stabilizedby way of a plate or a rod spanning the affected vertebrae. With thistechnique once fusion has occurred the hardware used to maintain thestability of the segment became superfluous. Moreover, the surgicalprocedures necessary to implant a rod or plate to stabilize the levelduring fusion were frequently lengthy and involved.

It was therefore determined that a more optimum solution to thestabilization of an excised disc space is to fuse the vertebrae betweentheir respective end plates, most optimally without the need foranterior or posterior plating. There have been an extensive number ofattempts to develop an acceptable intra-discal implant that could beused to replace a damaged disc and yet maintain the stability of thedisc interspace between the adjacent vertebrae, at least until completearthrodesis is achieved. These “interbody fusion devices” have takenmany forms, but many have had difficulty in achieving fusion, at leastwithout the aid of some additional stabilizing device, such as a rod orplate. Moreover, some of these devices are not structurally strongenough to support the heavy loads and bending moments applied at themost frequently fused vertebral levels, namely those in the lower lumbarspine.

The interbody fusion devices (IBFDs) that have overcome thesedifficulties are typically bulky, at least with respect to theintervertebral space. In particular, these devices have been configuredto completely fill the space and to restore the normal spinal anatomy atthe instrumented level. One drawback of this approach is that theimplant device is not exactly sized to the anatomy of the particularpatient, thus typically requiring pre-distraction of opposed vertebraein order to increase the disc space for device implantation. While acollection of differently sized IBFDs can be provided, it is unwieldyand impractical to provide an IBFD sized for every intervertebral discspace height.

Another drawback of these prior devices is that the surgical insertionsite must be at least as big as the IBFD. Minimally invasive and workingchannel surgical techniques have been recently developed that havesignificantly reduced the surgical invasion, but even more improvementis needed. The present invention provides an IBFD that achieves all ofthe benefits of prior IBFD designs, while also addressing theabove-noted drawbacks.

SUMMARY OF THE INVENTION

In order to address these drawbacks, the present invention contemplatesa device for distracting a body tissue space between opposing tissuesurfaces, comprising an upper plate having an outer surface configuredto contact one of the opposing surfaces and a lower plate having anouter surface configured to contact the other of the opposing surfaces,the lower plate having opposite side walls configured to removablysupport the upper plate thereon. The upper and lower plates combine todefine a cavity when the upper plate is supported on the lower plate.The lower plate includes a support surface for supporting at least oneexpansion member, or wafer, within the cavity, and a channelcommunicating with the cavity that is configured to receive an expansionmember conveyed therethrough for placement on the surface of the lowerplate.

The upper plate defines an upper surface for contacting an uppermostexpansion member within the cavity to displace the upper plate from thelower plate as additional expansion members are conveyed along thechannel. In one aspect of the invention, a releasable engagement featureis provided between the upper plate and the lower plate that isconfigured to hold the upper and lower plates together until the upperplate is displaced by the uppermost expansion member. In one embodiment,the releasable engagement feature includes at least one male element andcorresponding mating female element defined between the upper plate andthe lower plate. In this embodiment, the male element may be a ribdefined on each side wall of the lower plate and the female element maybe a corresponding recess.

The upper plate may be provided with a hub sized to fit between the sidewalls of the lower plate. The recess for the releasable engagementfeature then includes at least one groove defined on opposite sides ofthe hub that is configured for releasable engagement with a rib on acorresponding side wall of the lower plate. In one specific embodiment,the recess includes at least two grooves offset from each other onopposite sides of the hub, each of the grooves configured for releasableengagement with the rib on a corresponding side wall of the lower plate.

In another aspect of the invention, an expansion member is provided foruse with the expandable device that comprises a wafer sized to beconveyed through the channel and to be supported on the support surfaceof the lower plate. The wafer has opposite side walls configured to formpart of a releasable engagement feature between the wafer and the lowerplate. In certain embodiments, the wafer has opposite side walls, eachdefining a mating recess configured for releasable engagement with therib defined on each side wall of the lower plate.

In yet another feature of the invention, an expansion member forsequential insertion into a space between opposing tissue surfaces to bedistracted is provided that comprises an elongated body having an uppersurface and an opposite lower surface, and an insertion end and anopposite trailing end. A female feature is defined on one of the upperand lower surface, the female feature having an opening at the trailingend, while a male feature is defined on the other of the upper and lowersurface that is configured for insertion through the opening. Inaccordance with this embodiment, resilient interlocking features aredefined between the female and male features for resilientlyinterlocking adjacent elongated bodies when the male feature of one bodyis inserted into the female feature through the opening.

The female feature may be an elongated recess, while the male featuremay constitute an elongated boss configured to be received within therecess. The resilient interlocking features include at least one latchelement and indentation adjacent the latch element defined at oppositesides of the recess, and at least one corresponding mating indentationand mating latch element adjacent the mating indentation defined atopposite sides of the boss. This mating indentation and mating latchcombination is arranged so that the latch element is received in themating indentation and the mating latch element is received in theindentation when the elongated boss is received in the elongated recess.

In one particular embodiment, the resilient interlocking featuresincludes three of the latch elements and indentations spaced along thelength of the elongated recess, and three of the corresponding matingindentations and mating latch elements comparably spaced along thelength of the boss. The latch elements on the opposite sides of theelongated recess define a width therebetween, with the width decreasingbetween successive ones of the latch elements. Similarly, the matinglatch elements on the opposite sides of the elongated boss define awidth therebetween, with that width decreasing between successive onesof the mating latch elements.

In certain embodiments, the resilient interlocking features of the waferbody include a slot defined through the wafer extending along at least aportion of the length of the elongated boss. The slot is situatedbetween the mating indentation and the mating latch element on theopposite sides of the boss so that wafer may contract slightly as theinterlocking feature of one surface is pushed into engagement with theinterlocking feature of the opposite surface on an adjacent wafer as thewafers are sequentially inserted into the tissue space.

The resilient interlocking features lock sequential wafers againstrelative movement along the length of the wafers. In another aspect,features are provided that also lock sequential wafers against relativemovement perpendicular to their length. Thus, in one embodiment, thefemale feature is an elongated recess including the opening at one endand an end wall at end opposite the opening, the end wall defining arecess undercut. The male feature in this embodiment is an elongatedboss configured to be received within the recess and having a leadingboss defining a boss undercut arranged to interlock with the recessundercut when the boss is within the recess. The elongated recess mayalso define recess side undercuts in opposite sides of the recessadjacent the opening, and the elongated boss may define boss sideundercuts at opposite sides of the boss and arranged to interlock withthe recess side undercuts when the boss is within the recess.

In still another embodiment of the invention, an expansion member forsequential insertion into a space between opposing tissue surfaces to bedistracted comprises an elongated body having an upper surface and anopposite lower surface, and an insertion end and an opposite trailingend. A female feature is defined on one of the upper and lower surface,the female feature having an opening at the trailing end, while a malefeature is defined on the other of the upper and lower surface andconfigured for insertion through the opening. In this embodiment, alocking feature is defined between the female and male features forlocking adjacent elongated bodies against relative movement along thelength of the bodies when the male feature of one body is inserted intothe female feature through the opening. The locking feature preferablyincludes resiliently deformable elements defined on the female and malefeatures. The embodiment may also include an interlocking featuredefined between the female and male features for interlocking adjacentelongated bodies against relative movement perpendicular to the lengthof the bodies when the male feature of one body is inserted into thefemale feature through the opening.

An expansion member for sequential insertion into a space betweenopposing tissue surfaces to be distracted in another aspect of theinvention comprises an elongated body having an upper surface, anopposite lower surface, an insertion end and an opposite trailing end,wherein the body includes a leading boss projecting above the uppersurface adjacent the insertion end and a recess defined in the lowersurface beneath the boss, the leading boss defining a rear undercutopposite the insertion end. The body further defines a slot therethroughterminating at one end beneath the rear undercut and including at itsopposite end a flexible arm extending into the slot toward the boss andhaving an end positioned beneath the undercut. With this embodiment,when two of the expansion members are coupled, the boss of a lowermostexpansion member fits within the recess of the uppermost expansionmember, and the flexible arm of the uppermost expansion member istrapped between the undercut of the boss of the uppermost member and theboss of the lowermost expansion member.

In a further embodiment, an expandable interbody fusion device forimplantation into the intradiscal space between the opposing vertebralbodies in a spine comprises an upper plate having an outer surfaceconfigured to contact an upper vertebral body and a lower plate havingan outer surface configured to contact a lower vertebral body, a supportsurface and an opening communicating with the support surface. The upperplate and the lower plate are configured to be releasably engaged anddefine a cavity between the upper plate and the support surface of thelower plate. The fusion device further comprises at least two expansionmembers sized to be sequentially received through the opening onto thesupport surface, one expansion member beneath an immediately priorexpansion member to raise the immediately prior expansion member. In onefeature of this embodiment, a releasable engagement is defined betweenthe lower plate and each of the expansion members. This releasableengagement is operable to engage the immediately prior expansion memberraised by the one expansion member.

It is one object of the invention to provide an improved expandabledevice that may be used to distract the space between two body tissuesurfaces. A further object resides in aspects of the expandable devicethat allow for control expansion of the device, especially by sequentialinsertion of interlocking expansion members or wafers.

A further object of the invention is to provide expansion members thatinterlock in multiple degrees of freedom. One benefit of this feature isthat the wafers become interlocked upon sequential insertion, and do notbecome dislodged or disassociated with each other during expansion ofthe expandable device or after the device is complete in situ. Otherobjects and benefits of the invention will become apparent uponconsideration of the following written description taken together withthe accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a bottom perspective view of an interbody fusion device (IBFD)according to one embodiment of the invention.

FIG. 2 is a side view of the IBFD shown in FIG. 1.

FIG. 3 is a top view of the IBFD of FIGS. 1-2 mounted on an insertionapparatus in accordance with one aspect of the invention.

FIG. 4 is a side view of the IBFD and insertion apparatus shown in FIG.3.

FIGS. 5 a-5 f include perspective, side, end, top and bottom views of asuperior endplate portion of the IBFD shown in FIGS. 1-2, and includinga cross-sectional and enlarged view of portions thereof.

FIGS. 6 a-6 e include perspective, side, end, top and bottom views of aninferior endplate portion of the IBFD shown in FIGS. 1-2, including anenlarged view of a portion thereof.

FIGS. 7 a-7 e include side, top and cross-sectional views of theinferior endplate portion of the IBFD shown in FIGS. 6 a-6 e.

FIGS. 8 a-8 f include side, top, bottom and perspective views of a trackconnector used in connection with the insertion apparatus shown in FIGS.3-4, including cross-sectional views of portions thereof.

FIG. 8 g is a bottom perspective view of an alternative embodiment of atrack connector used in connection with the insertion apparatus shown inFIGS. 3-4.

FIG. 9 is a side perspective partial cut-away view of the IBFD andinsertion apparatus shown in FIGS. 3-4 with the track connector shown inFIG. 8 b in accordance with one embodiment of the invention.

FIG. 10 is a side view of the IBFD and insertion apparatus shown inFIGS. 3-4.

FIGS. 11 a, b are top perspective and bottom views of a wafer forintroduction into the IBFD of FIGS. 1-2 using the insertion apparatus asshown in FIGS. 3-4 and 9.

FIG. 12 is a side cut-away view of the structure shown in FIG. 9.

FIG. 13 is a top view of the distal end of the wafer-track portion ofthe insertion apparatus shown in the prior figures.

FIGS. 14 a-c are top, top perspective and top-perspective cut-away viewsof components of the insertion apparatus engaged with the inferiorendplate portion of the IBFD illustrated in FIGS. 6-7 and including thedistal end of the wafer track shown in FIG. 13.

FIG. 15 a is a top perspective view of a release plate, driver and thedistal end of the wafer track of FIG. 13.

FIG. 15 b is a top view of components of the insertion apparatus engagedwith the inferior endplate, including the release plate of FIG. 15 a.The track connector is removed to show the position of the release plateand the distal end of the wafer track in the inserter cavity.

FIG. 16 a is a bottom perspective view of the distal end of the wafertrack of FIG. 13 with the track connector of FIGS. 8 a, 8 b mountedthereon.

FIGS. 16 b-d are top, top perspective and top perspective cut-away viewsof components of the insertion apparatus engaged with the inferiorendplate portion and including the track connector of FIG. 8 b prior towafer insertion.

FIG. 17 is a top view of the insertion apparatus with a wafer situatedwithin the inferior endplate portion of the IBFD. The superior endplateis removed to show the position of the wafer in the wafer cavity.

FIG. 18 is a perspective cut-away view of the insertion apparatus, theinferior endplate portion of the IBFD, including the track connector,and wafer shown in FIG. 17.

FIG. 19 is a side pictorial view of the insertion apparatus being usedto insert an IBFD in accordance with the present invention into anintervertebral space.

FIGS. 20 a-20 c include side, top and end views of a disc spacedistractor for use with the insertion apparatus shown in the aboveidentified figures.

FIGS. 21 a-21 b are side and end cross-sectional views of an IBFD inaccordance with one embodiment of the present invention with a stack ofwafers introduced therein to one pre-determined height.

FIGS. 21 c-21 d are side and end cross-sectional views of the IBFD shownin FIGS. 21 a-21 b stacked to a different height in which all of thewafers are contained within the endplates.

FIGS. 22 a-d include side and end views of the IBFD shown in FIGS. 21a-21 b.

FIGS. 23 a-23 d include top and bottom perspective views, a side viewand a cross-sectional view of a superior endplate for a sagittallycurved embodiment of an IBFD of the present invention.

FIGS. 24 a-24 d include side, top perspective, top and end views of aninferior endplate for a sagittally curved embodiment of an IBFD of thepresent invention.

FIGS. 25 a-c are perspective, top and cross-sectional views of atransversely curved wafer for use with an IBFD of the present invention.

FIG. 26 is a side representation of an IBFD implanted in anintervertebral space with wafers as shown in FIGS. 25 a-c.

FIGS. 27 a-c are perspective, top and cross-sectional views of atransversely curved and angled wafer for use with an IBFD of the presentinvention.

FIG. 27 d is a side representation of an IBFD implanted in anintervertebral space with wafers as shown in FIGS. 27 a-c.

FIG. 28 is a bottom perspective view of an interlocking wafer accordingto a further embodiment of the invention.

FIG. 29 is a top perspective view of the interlocking wafer shown inFIG. 28.

FIG. 30 is an end elevational view of the interlocking wafer shown inFIGS. 28-29.

FIG. 31 is a bottom elevational view of the interlocking wafer shown inFIGS. 28-30 with a second wafer engaged thereto, as depicted in phantomliens.

FIG. 32 is a side elevational view of the interlocking wafer shown inFIGS. 28-30.

FIG. 33 is an enlarged partial side view of the region A of the wafershown in FIG. 32.

FIG. 34 is an enlarged partial side view of the region B of the wafershown in FIG. 32.

FIG. 35 is a bottom perspective view of an alternative configuration ofan interlocking wafer according to the present invention. FIG. 36 is atop perspective view of the alternative configuration shown in FIG. 35.

FIG. 37 is a top elevational view of the alternative configuration shownin FIGS. 35-36.

FIG. 38 is a bottom elevational view of the alternative configurationshown in FIGS. 35-36.

FIG. 39 is an enlarged partial side cross-sectional view of theinterlocking wafer depicted in FIG. 37 taken along line C-C as viewed inthe direction of the arrows.

FIG. 40 is a top elevational view of yet another alternativeconfiguration for an interlocking wafer according to the presentinvention.

FIG. 41 is a side elevational view of two wafers of the configurationshown in FIG. 40 depicted in their interlocking relationship.

FIG. 42 is a cross-sectional view of the two wafers illustrated in FIG.41.

FIG. 43 is a top perspective view of an expandable device configured toreceive a series of the wafers shown in prior figures.

FIG. 44 is an exploded view of the superior and inferior endplatecomponents of the expandable device shown in FIG. 43.

FIG. 45 is a top perspective view of the expandable device shown in FIG.43 engaged to a wafer inserter apparatus.

FIG. 46 is side perspective view of the expandable device depicted inFIG. 44, shown in an expanded configuration with a plurality ofinterlocking wafers disposed therein.

FIG. 47 is an end cross-sectional view of the expandable device andwafers shown in FIG. 46 taken along line E-E.

FIG. 48 is an end perspective cross-sectional view of the expandabledevice and wafers shown in FIG. 46 taken along line E-E.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

In accordance with one embodiment of the invention, an expandabledistraction device in the form of an interbody fusion device (IBFD) 10includes a superior endplate 12 and an inferior endplate 14 that definea wafer cavity 19, as shown in FIGS. 1-2. The superior and inferiorsurfaces of the endplates define engagement ribs 16 _(U) and 16 _(L)that are configured to engage or grip the vertebral endplates of opposedvertebrae in a spine. Preferably, the ribs 16 _(U) and 16 _(L) areconfigured to prevent expulsion of the IBFD under normal spinal loads.For instance, the ribs may have a saw tooth shape that is inclinedtoward the opening through which the IBFD is inserted into the interbodyspace. Angling the ribs toward the opening also angles them away fromthe direction of insertion so that the IBFD can be easily inserted intoa collapsed space.

The IBFD 10 also defines an inserter cavity 18 that engages a portion ofan inserter apparatus 50, as shown in FIGS. 3-4. The inserter apparatus50 defines a wafer track 52 along which a plurality of wafers, orexpansion members, are conveyed to fill the wafer cavity 19.

In accordance with one aspect of the invention, the IBFD 10 has a heightacross the superior and inferior endplates 12, 14 that is less than thenormal anatomic height of a typical intervertebral disc space. Theinvention contemplates that a series of expansion members, such aswafers, are introduced into the wafer cavity 19 to at least fill all orpart of the cavity, and to distract the opposing vertebrae by separatingthe superior and inferior endplates. Insertion of the wafers separatesthe endplates to expand the height of the IBFD within the intervertebralor interbody space and to ultimately restore the normal anatomic heightof the instrumented disc space.

Details of the superior and inferior endplates can be seen in FIGS. 5-7.Referring to FIGS. 5 a-5 f, and in particular to FIG. 5 d, the superiorendplate 12 includes an upper wall 22 on which the engagement ribs 16_(U) are defined. The interior face of the upper wall is thickened in areinforcement region 23. This region helps maintain the integrity of thesuperior endplate 12 and provides a strong surface against which alifting force can be applied by successive insertion of the wafer.Region 23 is also configured to contain and to cooperate with thewafers, as described below, to provide lateral and torsional stabilityto the wafer stack.

The upper wall terminates in an anatomically anterior end wall 24 and ananatomically posterior end wall 25 that integrate with the inferiorendplate 14 as described below. In addition, the reinforcement region 23defines outwardly and laterally projecting ribs 27 that engagecooperating notches 36 defined in the interior of the inferior endplate14. Details of the inferior endplate are shown in FIGS. 6-7. Theendplate 14 includes a bottom wall 30 on which the engagement ribs 16_(L) are defined. The bottom wall 30 terminates in an end wall 32 and aledge 33. As shown in FIGS. 2 and 9, the anterior end wall 24 of thesuperior endplate 12 overlaps the end wall 32 and end ledge 33 when theendplates are initially assembled. The two end walls 24 and 30 overlapover the majority of the height of the end wall 32 so that as thesuperior and inferior endplates are pushed apart the two endplatesremain in contact and continue to define the wafer cavity 19, providingstability to the IBFD as it expands.

The inferior endplate 14 also defines side walls 35 that define thewafer cavity and ultimately help retain the wafers within the cavity asthey are sequentially inserted. The inner face of the side walls definesnotches 36 that are aligned for engagement by the ribs 27 in thesuperior endplate 12. Thus, when the IBFD is initially assembled priorto insertion into the interbody space, the ribs and notches 27, 36 holdthe two endplates together. The interface between the ribs and notchesis adequate to hold the IBFD together as it is inserted into the space,but is sufficiently weak to be dislodged under pressure from theinserted wafers.

The interior of the inferior endplate 14 includes opposite surfaces 38that structurally reinforce the IBFD under large compressive loads.Slightly offset from the walls 38 are support rails 40 (FIG. 6 b) thatsupport the track connector 46 shown in FIGS. 8 a-8 f. The top surface49 of the track connector 46 is configured to be superior to surface 38such that any compressive load from the wafer stack is transmittedthrough the bottom surface of the track connector to the support rails40. The end walls 38 of the endplate 14 also form end notches 43 (FIG. 7c) that are complementary to the end edges of the track connector 46 inone embodiment of the invention. The end walls 38 and rails 40 of theendplate 14 define a connector channel 42, as shown in FIG. 7 a, whichis configured to receive the distal end of the wafer track of inserterapparatus 50, as described below.

The superior and inferior endplates 12, 14 may be formed of abiocompatible material with sufficient strength to support the adjacentvertebrae without fatigue and fracture. Preferably, the two endplatesare molded from a biocompatible polymeric material, such as, forexample, PEEK or a biocompatible composite material, such as, forexample carbon-fiber-reinforced PEEK. The material may also be selectedto permit tissue ingrowth to integrate with the vertebral endplates. Theendplates can further be formed from a moldable or formable biologicmaterial, such as bone.

In accordance with one aspect of this invention, the IBFD 10 isconfigured to be introduced into the interbody space by an introducer orinserter apparatus 50. The inserter can be constructed and operated likethe insertion apparatus disclosed in U.S. Pat. No. 6,595,998, entitled“Tissue Distraction Device”, which issued on Jul. 22, 2003, to theassignee of the present invention. The disclosure of this patent, andparticularly its discussion of the wafer inserter, is incorporatedherein by reference. Alternatively, the inserter can be constructed andoperated like the insertion apparatus disclosed in co-pendingapplication Ser. No. 10/813,819, entitled “Tissue Distraction Device”,filed on May 31, 2004, and assigned to the assignee of the presentinvention. The disclosure of this co-pending application is incorporatedherein by reference.

For purposes of illustration, certain details of the inserter 50 will beexplained herein. As shown in FIG. 3, the apparatus includes a wafertrack 52 along which wafers are conveyed to fill the wafer cavity 19within the IBFD and ultimately to expand the height of the IBFD. Oncethe last wafer has been introduced into the IBFD it is necessary toremove the inserter 50. The preferred embodiment of the inventioncontemplates a track connector 46 that helps to integrate the wafertrack 52 with the interior cavity of the IBFD and to provide a supportsurface for the wafer stack within the IBFD.

Details of the track connector 46 are shown in FIGS. 8 a-8 f and FIG. 9.In particular, the connector 46 includes connector posts 47 that projectdownward with the IBFD, as best seen in FIG. 9. These posts engagecorresponding openings 71 in an insertion plate 70 (see FIG. 12) toprovide an interface between the inserter apparatus 50 and the IBFD. Inone embodiment, the track connector 46 defines interface edges 48 at itsopposite ends that are configured to conform to wall 38 in the inferiorendplate 14 (see FIG. 6 b). The track connector may also include endedges 46 a flanking the interface edges that contact wall edges 38 a ofthe endplate 14 to limit the movement of the track connector into theendplate. The track support includes a ramp 49 a that helps directincoming wafers upward from the wafer track 52 to the wafer supportsurface 49 within the IBFD.

In an alternative embodiment shown in FIG. 8 g, a track connector 46′includes a modified proximal end 48′ and distal end 48″, but stillretains the connector posts 47, wafer support surface 49 and ramp 49 a.The modified distal end 48″ catches against a lip 39 formed in theinferior endplate, as shown in FIGS. 9, 12 to prevent removal of thetrack connector 46′ once it is positioned with the assembled IBFD. Thedistal end of the track connector 46′ further defines end edges 46 a′that contact the wall edges 38 a, as depicted in FIG. 16 b, in the samemanner as the end edges 46 a described above.

As shown in FIGS. 9, 10 and 12, the wafer inserter apparatus 50 providesan avenue for passage of wafers 55 from a wafer cartridge 54 into theIBFD. The inserter apparatus includes a cartridge gun that extractswafers 55 consecutively from a stack within the cartridge 54 and conveysthem along the track 52 to the IBFD. As shown in FIGS. 11 a-b, thewafers 55 are configured for transport along the track 52 and forinterlocking engagement within the IBFD. In particular, the wafersinclude a leading bevel 56 and an opposite trailing bevel 57 tofacilitate movement of each successive wafer underneath the immediatelyprior inserted wafer. The bevels 56, 57 help the incoming wafer dislodgeand slide underneath the wafer stack already resident within the IBFD.In certain embodiments, a wafer driver 65 may be provided within thewafer track 52 to advance each wafer into the wafer cavity. The driver65 can also help hold the lowermost wafer of the stack in position asthe inserter apparatus 50 is removed.

The wafers 55 also include interdigitating upper and lower surfaces 58,59, respectively. The surfaces can assume a variety of configurationsintended to prevent relative longitudinal movement between wafers in thestack as well as for lateral and rotational stability. The wafers 55 andtheir respective surfaces can be constructed as disclosed in U.S. Pat.No. 6,595,998 cited above. The disclosure of this patent, and mostparticularly its discussion of the construction of the wafers, isincorporated herein by reference. In the preferred embodiment, the uppersurface 58 defines a ridge 60 and spaced rib 61 extending along thelongitudinal axis of the wafer. Similarly, the lower surface defines alinear trough 62 that receives the ridge 60, and a notch 63 thatreceives the rib 61.

The insertion configuration for the IBFD and wafer inserter apparatus isgenerally depicted in FIG. 12. The wafer track 52 of the inserterapparatus engages the IBFD with the track end 53 contacting the proximalfaces of both the inferior endplate 14 and the superior endplate 12. Awafer 55 is shown resting on the wafer support surface 49 of the trackconnector 46′. The track connector 46 rests on the support rail 40 (seeFIG. 6) with its posts 47 projecting downward toward the post openings44 in the inferior endplate 14. As shown in the figures, the posts donot necessary extend into the openings 44. Instead, the post openings 44facilitate the assembly of insertion apparatus to the track connectorprior to use.

Beneath the track connector 46 reside an insertion plate 70 and arelease plate 75 immediately adjacent the connector 46. Both platesprovide openings to receive the connector posts 47 therethrough,including openings 71 in the insertion plate and openings 76 a-c in therelease plate. The insertion plate 70 may define a release track 72 (asshown in FIG. 14 c) within which the release plate 75 slides. Therelease track may be provided to increase the stiffness of the insertionplate, or may be eliminated to permit a reduction in width of thecomponents.

The assembly of the components of the inserter apparatus 50 within theIBFD 10 is depicted sequentially in FIGS. 13-18. The insertion plate 70is shown in FIG. 13. Preferably, the plate 70 is integral with the wafertrack 52. As shown in FIG. 12, the insertion plate 70 essentiallysupports the IBFD with the plate 70 extending into the wafer cavity andthe track end 53 abutting the IBFD. This plate 70 will be removed withthe inserter apparatus 50, leaving the IBFD within the interbody space.The post openings 71 are sized to receive the connector posts 47therethrough. As can be seen in FIGS. 14 a-c, the insertion plate 70sits below the support rail 40 in the inferior endplate 14 with its postopenings 71 aligned with the post openings 44 in the endplate 14.

The release plate 75, as shown in FIGS. 15 a-b, is slidably disposedwithin the release track 72 in the insertion plate 70. In an alternateembodiment, the release plate 75 is slidably disposed on top of theinsertion plate 70 without any release track 72. The release plate 75includes openings 76 a-c corresponding to each of the connector posts47. The distal edge 77 a-c of each opening is sharpened so that theywill sever the posts 47 from the connector plate 46 when the releaseplate is pulled proximally, or out of the IBFD. The opening 76 a isgenerally sized slightly larger than the post 47, while the other twoopenings 76 b-c are increasingly elongated. This configuration allowsthe distal-most post to be cleanly severed before the middle post issevered, and the middle post to be severed before the proximal post.This approach reduces the force needed to sever the posts. Once theposts are severed, they are retained within the post openings 71 via aninterference fit, since they are no longer needed to hold the trackconnector within the IBFD. When the posts are severed, the inserterapparatus 50 can be removed from the implanted IBFD without risk ofretracting the IBFD.

The next series of figures, FIGS. 16 a-d, show the placement of thetrack connector on top of the insertion plate 70 and release plate 75.As can be seen in FIG. 16 d, the wafer support surface 49 is generallycontiguous with wall 38 of the inferior endplate 14. In an alternateembodiment the wafer support surface 49 is superior to wall 38 of theinferior endplate 14. This alternate embodiment ensures that thecompressive load from the wafer stack is transmitted through the wafersupport surface 49 and not through wall 38. A first wafer 55 is added inFIGS. 17-18.

The inserter apparatus 50 and the IBFD 10 are shown in position forimplanting the IBFD within an interbody space. It is contemplated thatthe interbody or intradiscal space will be prepared in a known manner.In particular, the disc nucleus is removed by known means, preferablyleaving the disc annulus A relatively intact. A portal is formed in theannulus that is sized to the dimensions of the IBFD 10 in itsun-expanded configuration (as shown in FIGS. 1-2).

In the preferred arrangement, the IBFD is sized to be received in theunexpanded state through the portal into the disc space without anypre-distraction. In certain situations where the disc space height issmaller than the height of the unexpanded IBFD, pre-distraction may beused to slightly elevate the disc space so as to allow receipt of theunexpanded IBFD through the portal. Such pre-distraction, which canoccur using conventional techniques, is not intended to achieve thefinal disc space height. One approach is to use the distractor 80 shownin FIGS. 20 a-20 c. This distractor includes a distal end 82 having aheight H greater than its width W. The height H of the distal end 82 issubstantially constant over the insertion length L. The distractor isinserted into the disc space at a location adjacent to but laterallyspaced from the location where the IBFD is to be inserted with itslarger dimension parallel to the vertebral endplates. As such, nodistraction occurs during insertion of the distractor 80. The handle 84is used to rotate the distractor 80 until the larger dimension contactsand pushes apart the vertebral endplates. The distractor 80 can be heldin position as the IBFD is maneuvered into the interbody space using theinserter apparatus 50. After removal of the distractor, a second IBFDmay be inserted adjacent to the first implanted IBFD.

As shown in FIGS. 21 a-d and FIGS. 22 a-d, the IBFD can be expanded to aspecific height, with its height being determined by the number ofwafers 55 inserted into the IBFD. In the preferred embodiment, thesuperior and inferior endplates 12, 14 and the wafers have apre-determined height or thickness. As explained above, the endplatesinclude overlapping portions to help stabilize the stack, in particularthe end walls 24 and 32. After implanting the IBFD a biomaterial, suchas bone chips or other osteogenetic materials, such as bone morphogenicproteins or adipose-derived adult stromal cells, may be introducedadjacent to or in contact with the IBFD so as to promote fusion betweenthe opposing vertebrae.

As indicated in the figures, in certain embodiments of the invention,the stack height will change when the inserter apparatus is dislodgedfrom the IBFD and removed. In particular, the wafer stack will shiftslightly downward when the insertion plate and release plates areremoved, allowing the track connector 46 to drop down.

The IBFD 90 shown in FIGS. 21 a-d and FIGS. 22 a-d includes superior andinferior endplates 92, 94 that are angled. These endplates areconfigured to restore or maintain a particular angle of the vertebralmotion segment. For instance, if the IBFD 90 is used in the lumbarspine, the endplates are defined at a lordotic angle. The endplates 80,82 in FIGS. 23 a-d and FIGS. 24 a-d are also configured to have arcuateupper and lower surfaces for introduction into and anatomical support ofthe lumbar spine.

Alternative concepts for the endplates and the wafers are shown in FIGS.25 a-27 d. In FIGS. 25 a-c, a curved wafer 100 is provided. The waferincludes interlocking dovetail features 101 and 104 and locking notches102 to help hold the wafer stack together. As shown in FIG. 26, theendplates 105, 106 can be angled to restore the lordotic angle of themotion segment with the wafer stack therebetween.

As an alternative, the wafers can provide the lordotic angle, such asthe wafer 110 shown in FIGS. 27 a-c. The wafer 110 includes one end 111that is thicker than the opposite end 112. The wafers can be containedwithin endplates 115, 116 that are planar- i.e., that do not incorporatethe lordotic angle.

The wafers 55 shown in FIGS. 11 a-b include interlocking upper and lowersurfaces 58, 59. In particular, with this embodiment, the interlockingfeatures include a ridge 60 and rib 61 that are fed longitudinally intoa corresponding complementary shaped trough 62 and notch 63. With thisconfiguration, the stacked wafers resist dislodgement in the fore-aft(longitudinal) degree of freedom and resist relative rotation betweenadjacent wafers about a vertical axis extending through the stack. Thewafers 110 shown in FIGS. 27 a-c utilize a dovetail interface tointerlock adjacent wafers against vertical separation. Neither of theseprior embodiments provides a positive interlocking arrangement betweenadjacent wafers or complete interlocking in multiple degrees of freedom.

An expansion member 160 depicted in FIGS. 28-34 provides a positiveinterlock between adjacent expansion members that prevents dislodgementor separation in multiple degrees of freedom. As shown in the figures,the expansion member is in the form of an interlocking wafer 160 that isgenerally planar between the insertion end 162 and the trailing end 164.The insertion end 162 defines an upwardly facing beveled tip 163, whilethe trailing end defines a downwardly facing beveled tip 165. Thebeveled tips 163, 165 are configured to contact each other to push onewafer up as the other wafer is introduced into the expandable device,such as the IBFD 10 described above.

The wafer 160 provides interlocking features between the lower surface170 and the upper surface 190. These interlocking features areconfigured so that the wafers become positively interlocked as one waferis introduced beneath the next successive wafer as the wafer stack isformed. Ultimately, every wafer in the stack is positively interlockedwith the adjacent wafers above and below. Moreover, the interlockingfeatures are configured so that the interlocking elements mesh smoothlywithout raising and lowering the adjacent wafer during insertion of anew wafer.

In the preferred embodiment, the lower surface 170 of the wafer 160defines a recess 172 generally centered along the length or longitudinalaxis of the wafer. The recess 172 is open at the trailing end 164 but ispreferably closed at the insertion end 162, as shown in FIG. 28. Therecess 172 is bounded by opposite side walls 174 and an end wall 176.Part of the interlocking aspect of the wafer 160 is achieved by an entryundercut 175 defined in the side walls 174 at the open entry end of therecess (FIG. 33). The end wall 176 defines a similar undercut 177 (FIG.34).

The upper surface 190 of the wafer provides features that interlock withthe undercuts 175, 177. In the preferred embodiment, the upper surface190 includes a leading boss 192 at the insertion end 162 of the wafer,as shown in FIG. 29. The leading boss 192 defines an engaging undercut193 (FIG. 34) that is sized to fit snugly within the undercut 177 in theend wall 176 of the lower surface 170. The upper surface 190 furtherincludes a trailing boss 195 that extends from the trailing end 164toward the leading boss 192, but terminating short of the leading boss.The trailing boss 195 includes opposite side walls 196 that areconfigured for sliding contact with the side walls 174 of the recess 172in the lower surface of an adjacent wafer to help prevent relativerotational movement of stacked wafers. These opposite side walls formtrailing undercuts 202 (FIG. 34) that engage the undercuts 175 in theside walls of the lower surface recess.

Thus, when one wafer slides underneath a previously inserted wafer, thebeveled tip 163 at the insertion end 162 contacts the beveled tip 165 ofthe trailing end 164 of the previously inserted wafer, thereby liftingthat wafer to receive the newly inserted wafer. As the new wafer isinserted further, the leading boss 192 travels across the entry surface187 and enters the recess 172, followed by the trailing boss 195. As thenewly inserted wafer continues along the recess, the mating surface 204aligns with the entry surface 187, and the undercut 193 and undercut 202substantially simultaneously slide within the corresponding undercuts177 and 175 in the lower surface 170 of the previous wafer. Theseengagements between the undercuts in the lower and upper surfaces formpart of the interlocking connection between adjacent wafers 160.

A further aspect of the interlocking connection is achieved by resilientlatching elements between the lower and upper surfaces 170, 190. In thepreferred embodiment, each side wall 174 of the lower surface recess 172forms a latch element 180 followed by an indentation 182 (i.e., betweenthe latch element and the end wall 176 of the recess). Each side wall196 of the trailing boss 195 of the upper surface 190 formscomplementary latch elements 197 followed by indentations 199 (i.e.,between the upper surface latch elements and the trailing end 164 of thewafer). The latch elements 197 are particularly configured forengagement within the side wall indentations 182 in the lower surfacerecess 172. Likewise, the upper surface indentations 199 are configuredto receive the lower surface latch elements 180. This engagement isdepicted in FIG. 31, in which wafers 160 and 160′ (in phantom) areinterlocked with the latch elements 180 engaging the indentations 199′and the latch elements 197′ engaging the indentations 182. As can beseen from the figure the latch elements 180 and 197′ cooperate toprevent relative fore and aft movement between the wafers 160, 160′, aswell as relative rotational movement.

The leading boss 192 on the upper surface 190 is sized to pass betweenthe latch elements 180 on the lower surface of the adjacent wafer.However, in order for the interlocking feature to work, the latchelements 197 of the upper surface must have a normal engagementorientation that is wider than the recess 172 between the latch elements180 of the lower surface 170. Consequently, the present inventioncontemplates a resilient feature of the wafer 160 that allows resilientdeformation of one wafer relative to the other as the latch elementspass by each other. In accordance with this embodiment of the invention,the wafer defines a central slot 185 that passes between the lower andupper surfaces. The slot 185 preferably extends along a substantialportion of the length of the wafer, and most preferably has a lengthsufficient so that the latch elements 197 are positioned generally atthe mid-point of the length of the slot. With this configuration, theslot 185 has its region of maximum deformation where it is needed—at thelatch elements. Thus, when one wafer is inserted below a prior wafer,the slot 185 of the newly inserted wafer may constrict as the latchelement 197′ contacts the latch element 180. Once the latch element 197′reaches the indentation 182, the resilient nature of the wafer allowsthe slot 185 to spring back to its original width, thereby locking thelatch element 197′ within the indentation 182.

In a further feature of this embodiment, the wafer 160 includes a pairof pre-load recesses 208 on each side surface 206. The recesses engagecomplementary projections in the expandable device 250 (FIG. 44) to holdthe wafer in a predetermined position until dislodged from below.Details of this pre-load feature follow below in the discussion of theexpandable device 250.

A further embodiment of an interlocking wafer 215 is depicted in FIGS.35-38. This wafer 215 includes a lower surface 216, an upper surface 217and a thru slot 219 that are similar to the corresponding elements ofthe wafer 160. The lower surface 216 defines a recess 222 formed by sidewalls 223. The overall shape of the recess 222 is similar to the shapeof the recess 172, except that the single pair of latch elements in theprior embodiment is replaced by three pairs of latch elements 224, 225and 226.

The upper surface 217 includes a leading boss 228 and a trailing boss229 that are also similar to the like elements of the prior embodiment.The trailing boss 229 incorporates latch elements 231, 232 and 233 thatare configured to mate or interlock with the latch elements 224-226. Ascan be seen in FIGS. 37-38, the latch elements 224-226 and 231-233narrow toward the insertion end 220 of the wafer 215 so that the lateralspace between the forward latch elements 226 is narrower than betweenthe middle latch elements 225, which is narrower than the gap betweenthe trailing latch elements 224. This configuration produces aratcheting effect as a subsequent wafer interlocks with a previouswafer. In addition, the multiple latch elements ensure that adjacentwafers are firmly interlocked to prevent separation when the wafer stackis subjected to in situ forces.

While the latch elements interlock the wafers in the longitudinaldegrees of freedom, the wafer 215 also includes undercuts to interlockthe wafers in the vertical degree of freedom. In particular, in thisfurther embodiment, the recess 222 defines opposite undercuts 235 ainboard from the closed end of the recess, as shown in FIGS. 38-39. Theleading boss 228 of the upper surface 217 defines complementaryundercuts 236 a, as shown in FIGS. 37, 39, to mate with the undercut 235a when one wafer is fully inserted into the recess of a prior wafer.Similarly, the trailing end of the recess 222 defines opposite undercuts235 b while the trailing boss 229 defines complementary mating undercuts236 b, which are all similar to the like components on the wafer 160.

Yet another embodiment of an interlocking wafer 239 is illustrated inFIGS. 40-42. The lower surface 247 and upper surface 248 are configuredfor interlocking engagement, including insertion end mating undercuts246 (FIG. 42) and trailing mating undercuts 249 (FIG. 41). The wafer 239includes a leading boss 240 that defines an undercut 241. A wide slot243 is defined within the wafer from a point below the undercut 241 to alocation at the middle of the wafer. A trailing boss 242 is formed inalignment with the slot and includes a flexible portion or arm 244 thatis angled into the slot 243. The flexible portion 244 defines anundercut 245 at its tip that is arranged to contact the upper surface238 at the leading boss 240, as best shown in FIG. 42. When a subsequentwafer 239′ is inserted beneath a wafer 239, the leading boss 240′ pushesthe flexible arm 244 up into contact with the undercut 241 in theleading boss 240 of the upper wafer 239. The flexible arm 244 is thenwedged between the leading boss 240 of the upper wafer 239 and theleading boss 240′ of the lower wafer 239′ to interlock the wafers inmultiple degrees of freedom.

The wafer 160 of FIGS. 28-34, the wafer 215 of FIGS. 35-39, and thewafer 239 of FIGS. 40-42 each provide features for interlockingrelationship between adjacent wafers. Moreover, these features interlockthe wafers in several degrees of freedom, or against relativedisplacement in several directions. For example, the interlocking latchelements and indentations lock adjacent wafers in shear—i.e., relativemovement fore and aft, and side-to-side. The undercuts, 175, 177, 193and 202, for instance, resist fore-and-aft movement. In addition, therelationship between the latch elements and the undercut interfaceslocks the wafers in tension.

One important objective of interlocking in multiple degrees of freedomis to prevent dislodgement of adjacent wafers as a stack is beingformed. In certain embodiments, as new wafers are added, the height ofthe stack increases so that intermediate wafers of the stack are nolonger supported by the walls of the IBFD (such as IBFD 10). Theunsupported wafers may be susceptible to sliding apart or rotatingrelative to each other, which may disturb the integrity of the completedIBFD. Interlocking each wafer in the stack forms a substantially rigidstack that extends perpendicularly from the base of the stack upwardinto contact with the opposing surface. Moreover, interlocking thelowermost wafer to the next adjacent wafer helps hold that lowermostwafer against the insertion force of a newly inserted wafer.

Interlocking the lowermost wafer to the remainder of the stack alsohelps maintain the lowermost wafer in proper position to receive thenext wafer to be added to the stack. It is important that the lowermostwafer not be canted forward or backward. If canted forward, the trailingend 164 of the wafer 160, for instance, will block passage of the nextwafer to be introduced. If canted backward, the next wafer will notengage any of the interlocking features so that the prior wafer stackwill simply rest unconstrained on the upper surface 190 of the wafer.

As indicated above, each wafer may include pre-load recesses 208 on theside surfaces 206 of the wafers 160. These pre-load recesses are engagedby mating ribs in the inferior component of an expandable distractiondevice, such as the device 250 shown in FIGS. 43-46. The device 250includes a superior plate 251 and an inferior plate 252 that are similarin function to the like components of the device 10 described above. Theplates 251, 252 may include engagement ribs 254 that are configured toengage the opposing body tissue surfaces to be distracted. For instance,where the device 250 is used for interbody distraction, with or withoutfusion, the ribs 254 may be configured to engage the vertebralendplates. Other engagement or tissue gripping configurations may alsobe used, such as, for example, teeth, fins, ridges, threads and variouscombinations thereof. Additionally, porous surface coatings,indentations or openings may be used to promote bone ingrowth.

The superior and inferior plates 251, 252 are similar to the plates ofthe device 10 in that the plates are initially engaged during insertioninto the body space. Thus, in one embodiment, the side walls 253 of theinferior plate 252 define interior projecting ribs 265, while thesuperior plate 251 defines a series of mating grooves 268 on oppositesides of a lower hub 269 of the plate (FIGS. 44, 48). These ribs andgrooves form a releasable engagement feature that initially holds thetwo plates together, and that is configured to disengage upon pressurefrom the insertion of the expansion members or wafers. In thisembodiment, two rows of grooves 268 are provided, each row configuredfor a releasable snap-fit with the ribs 265. Thus, when the device 250is initially provided, as shown in FIG. 43, the superior plate 251 issnapped to the inferior plate 252, with the hub 269 disposed withinbetween the side walls 253 of the inferior plate. It should beunderstood that only one row of grooves 268 may be formed on the sidesof the lower hub 269 such that upon insertion of an initial expansionmember or wafer the ribs 265 release from the grooves 268 disengagingthe superior and inferior endplates 251, 252.

As shown in FIGS. 43-44, the side walls 253 of the inferior plate 252,together with front end wall 255 and rear end wall 259 form an open,upwardly facing full bounded cavity 261. The inferior plate 252 is openat one end to receive a wafer inserter or track, such as the track 52(see FIG. 45). Thus, the interior plate defines an insertion channel 256that includes a wafer channel 257 extending through rear end wall 259and through which successive wafers may be inserted, and an inserterchannel 258. The wafer channel 257 is defined in part by wafer supportledges 260 formed on the inside of each side wall 253, as shown in FIG.44. The ledges 260 provide a sliding surface for each new wafer beingintroduced through the track 52 into the device 250, as shown in FIGS.47-48.

The function of the pre-load recesses 208 in each wafer 160 is alsodepicted in FIGS. 47-48. In particular, it can be seen that thelowermost wafer rests on the ledges 260. The next adjacent wafer isrestrained by the ribs 265 engaged within the corresponding pre-loadrecesses 208 on each side of the device. As each new wafer is inserted,it displaces the previous wafer upward until the ribs and recessesinterlock. As the ribs and recesses interlock, the displaced waferassumes a stable and flat orientation so that the interlockingcomponents of the newly introduced wafer will align with the matinginterlocking components of the displaced wafer. Another benefit of theribs 265 and recesses 208 is that this snap-fit type engagement requiresa small load to dislodge or disengage. This pre-load may be easilyovercome by the introduction of a new wafer underneath the existingstack of wafers. However, the pre-load is sufficiently high that thestack cannot be inadvertently disengaged or moved upward by anatomicforces of extraneous forces occurring during the initial implantationprocess.

As also shown in FIGS. 47-48, the hub 269 of the superior plate 251includes wafer mating features 274 that are preferably identical to thefeatures on the lower surface 170 of each wafer 160, for example. Whenthe superior and inferior plates are initially assembled, the hub 269 isdisposed within cavity 261 and sits on or closely adjacent to the wafersupport ledges 260, with the ribs 265 engaged in the uppermost matinggroove 268. When the first wafer is introduced, the interlockingfeatures on the upper surface 190 of the wafer 160 engage the matingfeatures 274 on the underside of the boss 269 while the wafer lifts theboss, and therefore the superior plate 251, upwardly. Once the firstwafer is fully introduced into the inferior plate 252, the superiorplate has been lifted enough so that the lowermost groove 268, ifprovided, snaps into engagement with the ribs 265. If there is only onegroove 268, the superior plate 251 disengages from the inferior plate252 upon entry of the first wafer 160. As each successive wafer 160 isintroduced, this process repeats itself, with each new wafer becomingfully interlocked with the next preceding wafer. It should be noted thatthe two lowermost wafers 160, but at least the bottom wafer (as seen inFIGS. 47-48), reside within the cavity 261 of the inferior plate 252,fully contained and constricted by the opposing side walls 253 and thefront and rear end walls 255, 259.

This embodiment of the invention contemplates a mechanism for releasablyconnecting the expandable device 250 to the wafer track assembly 52.This mechanism incorporates a movable element that is operable todisengage the device from the track assembly. Thus, in one arrangement,the inferior plate 252 includes at least one, and most preferably three,upwardly projecting posts 262 (FIG. 44). The track assembly includes aninsertion plate 270 that is similar to the plate 70 (FIG. 12) of theprevious embodiment. The plate 270 defines a plurality of openings 271,each configured to receive a corresponding post 262 therethrough.Similar to the openings 71 in the plate 70 described above, the openings271 in the insertion plate 270 may be provided with a cutting edge thatis configured to sever the posts 262 when the plate 270 is retractedwithin the track assembly 52. When the posts are severed, the connectionbetween the inferior plate 252 and the insertion plate 270 is broken,which ultimately releases the inferior plate and device 250 from thetrack assembly 52. Once the inferior plate is disengaged, the trackassembly may be removed while the device 250 is left in situ.

It is understood that the track assembly 52 may be used to help maintainthe inferior plate 250 in position between the body tissues to bedistracted. Once disengaged, the inferior plate remains within thespace, so that the inferior plate essentially acts as a base wafer uponwhich the wafer stack is built. It is therefore important for theinferior plate 252, acting as a base wafer, to remain stable andsteadfast in contact with the inferior tissue surface. When the device250 is used as an interbody device, the inferior plate 252 must remainin solid, fixed contact with the inferior vertebral endplate, assistedby the engagement ribs 254, or other gripping surfaces or bone ingrowthfeatures, as described above. In one preferred procedure, the opposingendplates are scraped or otherwise reduced to bleeding bone to enhancethe temporary and permanent fixation of the device plates 251 and 252 tothe adjacent vertebral bodies.

It is contemplated that in certain applications, the tissue space to bedistracted will be expanded by expansion of the device 250. Thus, witheach new wafer inserted, the force needed to push the wafer underneaththe immediately prior wafer may be increased by the force needed todistract the body space. However, if the space to be distracted is lessthan the un-expanded height of the device 250 (as illustrated in FIG.43), additional distraction may be required. In that case, a separatedistraction instrument may be introduced into the body space and used totemporarily distract the space enough to allow insertion of the device250. The distractor 80 shown in FIG. 20 may be used to open the spacesufficiently for insertion of the device. As the tip is rotated, itforces the opposing tissue surfaces apart.

It is contemplated that the distraction tool 80 would be introducedtoward one side of the space to leave adequate room within the heart ofthe space to receive the expandable device 250. In some embodiments, anadditional distraction tool may be used at an opposite side of thespace. Once the distraction device has been inserted, the distractiontool 80 may then be removed from the space. In certain procedures, awider distraction tool tip may be used to temporarily expand the spaceto the intended distracted height. In this approach, the expandabledevice 250 would be expanded until the device spanned the pre-distractedspace, at which time the distraction tool(s) may be removed.

In accordance with certain specific embodiments, the device 250 has anon-expanded height of about 6 mm at its leading (or insertion) end. Asshown in FIG. 46, the tissue engaging surfaces of the superior andinferior plates 251, 252 are preferably domed in specific embodimentsintended for use in distracting an intervertebral space. Thenon-expanded domed device then has a maximum height in the center ofabout 8.5 mm with the engagement ribs. The maximum height of anon-expanded flat device with engagement ribs would be about 7.0 mm overits length. The overall length and width of the device may be calibratedto substantially fill the intervertebral space, or may be preferablysized to leave room around the device for the introduction of fillermaterial around the device. For instance, in one embodiment, the device250 has a length of about 24 mm and a width of about 9 mm, which leavessufficient space around the device and within an intact intervertebraldisc to pack an osteo-inductive and/or osteo-conductive material. Thematerial may be, for instance, bone chips that are fed into the spacearound the device 250 using a minimally invasive cannula.

As described above, the interlocking wafers 160, 215 or 239 areconfigured to add about 1.0 mm to the expanded height of the device 250.For instance; for the wafer 239 shown in FIG. 39, the height h is about1.0 mm. It is understood that the overall height of the wafer will begreater than the expansion height to accommodate the mating undercuts235 and 236. The width and length of the wafers is dictated by theinterior dimensions of the inferior plate 252 where the wafers areinitially introduced into the wafer stack. In a specific embodiment, thewafers have a width of about 7.7 mm and a length of about 20 mm to fitwithin a correspondingly sized space in the inferior plate 252.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected. For instance, while theillustrated embodiments have been directed to interbody fusion of thespine, the expandable devices and wafers disclosed herein may be used inother applications that require distraction of tissue surfaces.Modifications in size may be necessary depending upon the body spacebeing distracted.

1. A device for distracting a body tissue space between opposing tissuesurfaces, comprising: an upper plate having an outer surface configuredto contact one of the opposing surfaces; a lower plate having an outersurface configured to contact the other of the opposing surfaces, saidlower plate having opposite side walls configured to removably supportsaid upper plate thereon; said upper and lower plates defining a cavitywhen said upper plate is supported on said lower plate, said lower platedefining a support surface for supporting at least one expansion memberwithin said cavity, and a channel communicating with said cavity andconfigured to receive an expansion member conveyed therethrough forplacement on said surface of said lower plate; said upper plate defininga contact surface for contacting an uppermost expansion member withinsaid cavity to displace said upper plate from said lower plate asadditional expansion members are conveyed along said channel, saidcontact surface having a mating interface for interlocking engagementwith a complementary mating interface on said uppermost expansionmember; and a releasable engagement feature between said upper plate andsaid lower plate configured to hold said upper and lower plates togetheruntil said upper plate is displaced by the uppermost expansion member.2. The device of claim 1, wherein said releasable engagement featureincludes at least one male element and corresponding mating femaleelement defined between said upper plate and said lower plate.
 3. Thedevice of claim 2, wherein said male element is a rib and said femaleelement is a corresponding recess.
 4. The device of claim 2, whereinsaid rib is defined on each side wall of said lower plate and saidcorresponding recess is defined in said upper plate.
 5. The device ofclaim 4, wherein: said upper plate includes a hub sized to fit betweensaid side walls of said lower plate; and said corresponding recessincludes at least one groove defined on opposite sides of said hub andconfigured for releasable engagement with a rib on a corresponding sidewall.
 6. The device of claim 5, wherein said corresponding recessincludes at least two grooves offset from each other on opposite sidesof said hub, each of said grooves configured for releasable engagementwith a rib on a corresponding side wall.
 7. The device of claim 4,wherein at least a pair of ribs is defined on each side wall, each ofsaid pair of ribs arranged for releasable engagement with said at leastone groove.
 8. The device of claim 1, further comprising a wafer sizedto be conveyed through the channel and to be supported on the supportsurface of the lower plate, said wafer having opposite side wallsconfigured to form part of a releasable engagement feature between saidwafer and the lower plate.
 9. The device of claim 4, further comprisinga wafer sized to be conveyed through the channel and to be supported onthe support surface of the lower plate, said wafer having opposite sidewalls each defining a mating recess configured for releasable engagementwith the rib defined on each side wall of the lower plate.
 10. Anexpansion member for sequential insertion into a space between opposingtissue surfaces to be distracted, comprising: an elongated body havingan upper surface and an opposite lower surface, and an insertion end andan opposite trailing end; a female feature defined on one of said upperand lower surface, said female feature having an opening at saidtrailing end; and a male feature defined on the other of said upper andlower surface and configured for insertion through said opening; andresilient interlocking features defined between said female and malefeatures for resiliently interlocking adjacent elongated bodies when themale feature of one body is inserted into said female feature throughsaid opening.
 11. The expansion member of claim 10, wherein: said femalefeature is an elongated recess; said male feature is an elongated bossconfigured to be received within said recess; and said resilientinterlocking features include; at least one latch element andindentation adjacent said latch element defined at opposite sides ofsaid recess; and at least one corresponding mating indentation andmating latch element adjacent said mating indentation defined atopposite sides of said boss and arranged so that said latch element isreceived in said mating indentation and said mating latch element isreceived in said indentation when said elongated boss is received insaid elongated recess.
 12. The expansion member of claim 11, whereinsaid resilient interlocking features includes three of said latchelements and indentations spaced along the length of said elongatedrecess, and three of said corresponding mating indentations and matinglatch elements comparably spaced along the length of said boss.
 13. Theexpansion member of claim 12, wherein said latch elements on saidopposite sides of said elongated recess define a width therebetween,said width decreasing between successive ones of said latch elements.14. The expansion member of claim 13, wherein said mating latch elementson said opposite sides of said elongated boss define a widththerebetween, said width decreasing between successive ones of saidmating latch elements.
 15. The expansion member of claim 11, whereinsaid resilient interlocking features includes a slot defined throughsaid wafer extending along at least a portion of the length of saidelongated boss and disposed between said mating indentation and saidmating latch element on said opposite sides of said boss.
 16. Theexpansion member of claim 15, wherein said slot is a closed slotextending along substantially the entire length of said recess.
 17. Theexpansion member of claim 10, wherein: said female feature is anelongated recess including said opening at one end and an end wall atend opposite said opening, said end wall defining a recess undercut; andsaid male feature is an elongated boss configured to be received withinsaid recess and having a leading boss defining a boss undercut arrangedto interlock with said recess undercut when said boss is within saidrecess.
 18. The expansion member of claim 17, wherein: said elongatedrecess defines recess side undercuts in opposite sides of said recessadjacent said opening; and said elongated boss defines boss sideundercuts at opposite sides of said boss and arranged to interlock withsaid recess side undercuts when said boss is within said recess.
 19. Theexpansion member of claim 10, wherein said insertion end is beveled. 20.The expansion member of claim 19, wherein said trailing end is beveledto coincide with the bevel of said insertion end when one expansionmember is inserted into the tissue space beneath another expansionmember.
 21. An expansion member for sequential insertion into a spacebetween opposing tissue surfaces to be distracted, comprising: anelongated body having an upper surface and an opposite lower surface,and an insertion end and an opposite trailing end; a female featuredefined on one of said upper and lower surface, said female featurehaving an opening at said trailing end; and a male feature defined onthe other of said upper and lower surface and configured for insertionthrough said opening; and a locking feature defined between said femaleand male features for locking adjacent elongated bodies against relativemovement along the length of the bodies when the male feature of onebody is inserted into said female feature through said opening.
 22. Theexpansion member of claim 21, wherein said locking feature includesresiliently deformable elements defined on said female and malefeatures.
 23. The expansion member of claim 22, wherein: said femalefeature is an elongated recess; said male feature is an elongated bossconfigured to be received within said recess; and said resilientlydeformable elements include; at least one latch element and indentationadjacent said latch element defined at opposite sides of said recess;and at least one corresponding mating indentation and mating latchelement adjacent said mating indentation defined at opposite sides ofsaid boss and arranged so that said latch element is received in saidmating indentation and said mating latch element is received in saidindentation when said elongated boss is received in said elongatedrecess.
 24. The expansion member of claim 21, further comprising aninterlocking feature defined between said female and male features forinterlocking adjacent elongated bodies against relative movementperpendicular to the length of the bodies when the male feature of onebody is inserted into said female feature through said opening.
 25. Theexpansion member of claim 24, wherein: said female feature is anelongated recess including said opening at one end and an end wall atend opposite said opening, said end wall defining a recess undercut; andsaid male feature is an elongated boss configured to be received withinsaid recess and having a leading boss defining a boss undercut arrangedto interlock with said recess undercut when said boss is within saidrecess.
 26. The expansion member of claim 25, wherein: said elongatedrecess defines recess side undercuts in opposite sides of said recessadjacent said opening; and said elongated boss defines boss sideundercuts at opposite sides of said boss and arranged to interlock withsaid recess side undercuts when said boss is within said recess.
 27. Anexpansion member for sequential insertion into a space between opposingtissue surfaces to be distracted, comprising: an elongated body havingan upper surface, an opposite lower surface, an insertion end and anopposite trailing end; said body including a leading boss projectingabove said upper surface adjacent said insertion end and a recessdefined in said lower surface beneath said boss, said leading bossdefining a rear undercut opposite said insertion end; said body furtherdefining a slot therethrough terminating at one end beneath said rearundercut and including at its opposite end a flexible arm extending intosaid slot toward said boss and having an end positioned beneath saidundercut, whereby when two of said expansion members are coupled, saidboss of a lowermost expansion member fits within said recess of theuppermost expansion member, and said flexible arm of the uppermostexpansion member is trapped between said undercut of said boss of theuppermost member and said boss of the lowermost expansion member. 28.The expansion member of claim 27, wherein said recess defines a recessundercut and said boss defines a leading undercut configured to engagesaid recess undercut when two expansion members are coupled.
 29. Theexpansion member of claim 27, further comprising: a rear boss projectingabove said upper surface, said rear boss defining a forward undercut;and a rear recess defined in said lower surface beneath said rear boss,said rear recess defining a rear recess undercut configured to engagesaid forward undercut when two expansion members are coupled.
 30. Anexpandable interbody fusion device for implantation into the intradiscalspace between two opposing vertebral bodies of a spine, comprising: asuperior endplate member having an outer surface for engaging a superiorvertebral body in a spine; an inferior endplate member having an outersurface for engaging an inferior vertebral body in said spine, saidsuperior endplate member and said inferior endplate member beingreleasably coupled and defining a cavity therebetween; at least oneexpansion member configured to be introduced into said cavity to movesaid endplate and said inferior endplate member relatively apart uponintroduction and to thereby decouple said superior endplate member andsaid inferior endplate member; and an interlocking interface defined bycomplementary mating configurations on said expansion member and asurface on one of said superior endplate member or said inferiorendplate member to provide interlocking engagement therebetween.
 31. Thefusion device of claim 30, wherein said interlocking interface isdefined between a surface on said superior endplate member and saidexpansion member.
 32. The fusion device of claim 31, wherein saidexpansion member is defined by a generally flat wafer having an uppersurface and an opposing lower surface.
 33. The fusion device of claim32, wherein the complementary mating configuration on said superiorendplate member is defined by a slot formed into a lower surface of saidsuperior endplate member and said complementary mating configuration onsaid wafer is defined by at least one boss projecting from the uppersurface of said wafer and configured for complementary sliding fitwithin said slot.
 34. The fusion device of claim 33, wherein said bossis formed to provide a flexible latch operable upon insertion into saidslot.
 35. The fusion device of claim 34, wherein said complementarymating configuration on said superior endplate member is defined by afirst undercut surface and said complementary mating configuration onsaid wafer is defined by a second undercut surface configured to engagesaid first undercut surface upon insertion of said boss into said slot.36. An expandable interbody fusion device for implantation into theintradiscal space between the opposing vertebral bodies in a spine,comprising: an upper plate having an outer surface configured to contactan upper vertebral body; a lower plate having an outer surfaceconfigured to contact a lower vertebral body; said upper plate and saidlower plate configured to be releasably engaged and defining a cavitytherebetween when engaged; at least one expansion member sized to bereceived into said cavity and upon receipt therein to disengage saidupper and lower plates; said expansion member being configured to forman interlocking interface with said upper plate; and said expansionmember being configured to provide a releasable engagement with saidlower plate upon receipt of a further expansion member into said cavity.37. The fusion device of claim 36, wherein said lower plate has a pairof opposing spaced apart sidewalls.
 38. The fusion device of claim 37,wherein said upper plate and said lower plate are releasably engaged bya groove on said upper plate and a corresponding rib on at least one ofsaid sidewalls.
 39. The fusion device of claim 38, wherein saidexpansion member is defined by a generally flat wafer having an uppersurface and an opposing lower surface with side edges therebetween. 40.The fusion device of claim 39, wherein said wafer and said lower plateare releasably engaged by a recess formed into a side edge of said waferand said rib on at least one of said sidewalls of said lower plate. 41.The fusion device of claim 40, wherein said lower plate further includesa front end wall and a spaced opposing rear end wall that together withsaid sidewalls form said cavity fully bounded thereby.
 42. The fusiondevice of claim 41, wherein said interlocking interface is defined by amating configuration on a lower surface of said upper plate and acomplementary mating configuration on the upper surface of said wafer.43. The fusion device of claim 42, comprising at least one further waferreceived between said at least one wafer and said further wafer definingan interlocking interface therebetween, said further wafer beingcontained within the sidewalls and the front and rear end walls of saidlower member.
 44. An expandable interbody fusion device for implantationinto the intradiscal space between the opposing vertebral bodies in aspine, comprising: an upper plate having an outer surface configured tocontact an upper vertebral body; a lower plate having an outer surfaceconfigured to contact a lower vertebral body, a support surface and anopening communicating with said support surface; said upper plate andsaid lower plate configured to be releasably engaged and defining acavity between said upper plate and said support surface; at least twoexpansion members sized to be sequentially received through said openingonto said support surface, one expansion member beneath an immediatelyprior expansion member to raise said immediately prior expansion member;and a releasable engagement defined between said lower plate and each ofsaid expansion members, said releasable engagement operable to engagesaid immediately prior expansion member raised by said one expansionmember.
 45. The fusion device of claim 44, wherein: said lower plateincludes side walls at opposite sides of said cavity; each of saidexpansion members includes opposite side edges positioned adjacent saidopposite sides when said expansion members are within said cavity; andsaid releasable engagement includes a recess formed in at least one sideedge of said expansion member and a mating rib projecting from at leastone of said side walls of said lower plate.